Various scale grating encoders that use optical receiver channels are known, including, for example, those miniature fiber optic encoders disclosed in U.S. Pat. Nos. 6,906,315; 7,053,362; 7,126,696; and 7,973,941 (the '315, '362, '696 and '941 patents), each of which is hereby incorporated herein by reference in its entirety. Such encoders offer a desirable combination of features which may include extremely small size, very high accuracy, electrical noise immunity, and very high-speed operation.
In various applications, it is desirable to use an encoder which is robust to signal degradation resulting from contamination on a scale track. An encoder disclosed in U.S. Pat. No. 7,701,593 (the '593 patent) which is hereby incorporated by reference in its entirety, uses three different wavelengths of source light to determine a position. Each of the three wavelengths is spatially filtered in the readhead using a periodic optical element which acts as three periodically interleaved etalons to provide three distinct spatial phase signals. Such an encoder is robust to contamination or signal drift due to certain thermal effects. However, such a periodic optical element must be manufactured to have very precise separation between the faces of the respective “strip-like” etalons, while at the same time, periodically interleaving the etalons along the measuring axis directions. This requires a very high level of precision in manufacturing such a device. In addition, installation may require a small gap (<100 um). An encoder which is robust to contamination or signal degradation due to thermal effects and is easy to manufacture and install is desirable.